The thing with rodents is.....the longer you keep them...the more money they cost to feed and the more space they take up....

If you want to turn over at least 6 adult rats a week.....lets do some calculations

Days needed for gestation = around 22 days
Days until weaned = around 22-25 days
Days until medium-large size is reached = around 40-60 days

So that is around 44-48 days until a female will be available to breed again because she is either pregnant or nursing. In order to produce around 6+ rats a week, you would need to have around 1 litter every week maybe 1 every two weeks if you have good producers. If a female is not available for 40-45 days (around 7 weeks), I would go with around 8-9 females. If you dont have enough babies...get some more females....if you have too many babies, feed some of the breeders.

You would need a place to house all the breeder and the babies once they are weaned.